<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            /* 假如浏览器窗口的大小是750px */
            /* 100px */
            font-size: calc(100vw / 7.5);
        }

        p {
            /* 后面所有的宽高都用rem  /100 */
            font-size: .3rem;
        }
    </style>
</head>

<body>

    <div class="a">
        <p>啊啊啊</p>
    </div>

</body>

</html>